Semiconductor wafers are a fundamental component of the electronic industry, and improving the quality and efficiency of their processing is, therefore, highly desirable. Wafers are processed in sealed vacuum chambers, where they are supported in a processing machine by a chuck, through which cooling gas flows to contact the bottom surface of the wafer. The wafer, therefore, has a different pressure environment on its top and bottom surfaces.
For procedures in which holes are etched nearly or completely through the wafer, termed very deep to through-wafer etching, the etch must be stopped before it reaches the chuck. Holes that reach the bottom of the wafer expose the chuck to the process environment, damaging the chuck and contaminating the chamber. To prevent this, a thin film layer is placed on the bottom surface of the wafer to stop the etch. This solution causes further problems, because the thin film sticks to the chuck when heated. The thin films used also do not adequately support the wafer structure during and after very deep to through-wafer etching, causing wafers to cleave or shatter during processing.
An additional constraint introduced by very deep to through-wafer etching arises during removal of the wafer from the chuck. Typically, pins rise out of the chuck to lift the wafer and a spatula reaches underneath the wafer to move it out of the process chamber. Deep holes are problematic if they are in the path of the pins or if they significantly decrease the wafer's structural integrity.
A current solution is to use a backing wafer adhered to the process wafer with a thin film, for example photoresist, sandwiched between the two wafers. The backing wafer system, however, introduces further problems. Physical pressure on the process wafer and elevated temperatures are needed to effect adhesion, complicating processing and introducing significant potential for contamination. When the wafer is inserted into the processing chamber, air bubbles between the process and backing wafers, poor quality adhesion, or delaminating of the process wafer at high temperatures cause the wafers to break violently. Wafer breakage is a catastrophic and costly event, requiring operators to shut down and clean the process equipment.
What is needed is a device, such as a wafer holder, that can be used to protect the chuck and support the substrate during through-wafer etching without using a backing wafer.
Existing wafer holders are not designed for very deep to through-wafer etching and do not address all of the requirements outlined above. In general they are fixed to the chuck and themselves need to be protected from contamination by the process environment.
In U.S. Pat. No. 4,213,698, Firtion et al. disclose an apparatus for holding a workpiece during semiconductor processing. Their device creates a planar holding face on the ends of many closely-spaced pins. The apparatus is not applicable for very deep to through-wafer etching and does not allow for cooling gas flow through the device to the wafer.
Hattori describes a substrate carrier in U.S. Pat. No. 4,646,418. The carrier is designed to minimize operator handling of the substrate and is not used during processing.
A workpiece carrier for heat transfer under vacuum conditions is described by Wagner et al. in U.S. Pat. Nos. 5,033,538 and 5,180,000. The carrier contains a complex system of channels and grooves through which a heat transfer gas flows. The channel system is highly complicated, and the carrier is not suitable for through-wafer etching.
Finally, in U.S. Pat. No. 4,846,452 Geneczko discloses a rotational chuck assembly for finely controlling the rotational position of a wafer on a chuck. The assembly is very mechanically complicated, and is actually part of a chuck, not a separate wafer support used to protect a chuck.